In a typical optical communication system, an optical transmitter generates an optical beam and modulates the beam with an electrical signal representative of the information to be transmitted by the communication system. An optical fiber propagates the modulated optical signal to a receiver that demodulates the optical beam to recover the electrical signal. Fiber amplifiers, disposed at appropriate intervals in the fiber optic link between the transmitter and the receiver, maintain the strength of the optical signals. The low loss, light weight, small size, flexibility and high intrinsic bandwidth of optical fiber make optical communication systems highly desirable for the communication of both of digital and analog signals. The many applications of optical communication systems include cable TV (CATV) systems, telephone and other cross-country or cross-continent communication systems, and microwave and RF, such as phased array antenna systems used by the military, where the optical communication system can replace the complex and bulky down converters and up converters typically located at the front end of the microwave or RF antennas.
One important concern with optical communication systems, and in particular in systems that employ Dense Wavelength Division Multiplexing (DWDM) techniques to increase information carrying capacity, is the monitoring and control of the wavelength of the laser source. The wavelength of a typical laser source is known to be affected by several factors, such as laser source current, laser temperature, and aging of the laser, all leading to variations in the laser wavelength, which can affect the performance of other system components and affect overall system performance. For example, in a DWDM system, multiple beams, each of a different wavelength and representing a distinct channel for the transmission of data, are combined (multiplexed) to propagate as a beam along a single optical fiber. In an DWDM system, the wavelength stability of the laser sources limits number of channelsxe2x80x94the channels cannot be so closely spaced such that the wavelength of one channel laser source drifts too close to the wavelength at which another channel light source is operating. Information will be lost. Accordingly, the better the regulation of the wavelength of the laser sources, the more densely the channels can be packed within a particular wavelength range.
One method for regulating the wavelength of the laser is to regulate the temperature of the laser. For example, Distributed Feedback (DFB) lasers are typically temperature stabilized using a thermal control loop consisting of a thermistor to sense the device temperature, an electronic feedback loop, and a thermoelectric cooler (TEC) that responsive to feedback adjusts the temperature of the laser. Thermal regulation is employed because it also protects the DFB laser from overheating, and helps to stabilize power output of the laser. However, laser drift is still a concern and limits the density of channels.
Known in the art are closed loop systems that monitor the actual wavelength of the laser and provide an error signal responsive to the deviation of the laser wavelength from a desired wavelength, and use the error signal to control the temperature or excitation current of laser as a means of controlling the laser wavelength. These systems can be an improvement over a system that just regulates the temperature of the laser. The error signal can control the temperature of the laser or the current supplied to the laser. However, the monitoring apparatus itself can also be sensitive to temperature changes, and compensation is best made for this temperature sensitivity. Unfortunately, currently practiced techniques for compensating for the temperature sensitively of the wavelength monitoring apparatus can be cumbersome to implement and/or produce less than satisfactory results. Improvement in such techniques can also lead better system performance via improved wavelength management and optimization.
Accordingly, it is an object of the invention to provide improved monitoring and/or control of laser wavelength, in particular by providing methods and apparatus for compensating for the effects of temperature variation of the wavelength monitoring apparatus.
Other objects of the invention will in part be apparent and in part appear hereinafter.
In one aspect, the invention provides an apparatus for monitoring the wavelength of laser radiation. The apparatus includes an optical filter for receiving at least a portion of the laser radiation and for transmitting a first filtered beam in accordance with a first spectral filter function and for reflecting a second filtered beam in accordance with a second spectral filter function. The spectral filter functions cross at least one crossing wavelength. Also included are: first and second optical detectors for receiving the first and second filtered beams, respectively, and for providing first and second detected signals; temperature sensor for sensing a temperature characteristic of at least the optical filter; and processing circuitry for providing a temperature-corrected error signal responsive to the deviation of the wavelength of the laser radiation from a nominal wavelength. The processing circuitry includes: an error circuit for providing, responsive to the first and second detected signals, an uncorrected error signal responsive to the deviation of the wavelength of the laser radiation from the nominal wavelength; and a memory for providing offset values corresponding to selected temperatures, and wherein the processing circuitry, responsive to the temperature sensor, modifies the uncorrected error signal based on at least one offset value to produce the temperature-corrected output signal.
The optical filter can mount the first and second optical detectors, and the processing circuitry can include a microcontroller chip that includes on the chip the memory and the error circuit, the memory including a PROM circuit. Furthermore, the microcontroller can be programmed such that the determination of the error signal by the error circuit includes determining a ratio of the difference between the detected signals to the sum of the detected signals, and also for performing self calibration for determining and storing the offset values in the memory. The self calibration includes, at selected temperatures, determining the deviation of the uncorrected error signal from a predetermined value and storing an offset responsive to the deviation and associating the stored offset with one of the selected temperatures.
In another aspect of the invention, apparatus for monitoring the wavelength of laser radiation includes at least one optical filter for filtering the laser radiation according to at least one spectral filter function to produce filtered laser radiation; at least one optical detector for detecting the filtered laser radiation to produce a first detected signal; a temperature sensor for sensing a temperature characteristic of at least the optical filter; and processing circuitry for providing a temperature-corrected output signal responsive to the deviation of the wavelength of the laser radiation from a nominal wavelength. The processing circuitry can include an error circuit for providing, responsive to at least the first detected signal, an uncorrected signal responsive the deviation of the wavelength of the laser radiation from the nominal wavelength; and a memory for providing offset values corresponding to selected temperatures, and wherein the processing circuitry, responsive to the temperature sensor, modifies the uncorrected signal based on at least one offset value to produce the temperature-corrected output signal.
In yet a further aspect, the invention includes a laser apparatus incorporating temperature-corrected wavelength monitoring apparatus, such as that described above. Also included is a laser and a laser wavelength control responsive to temperature-corrected error signals. The laser wavelength control can control the current or voltage supplied to the laser or the temperature of the laser. A thermoelectric cooler can be included for controlling the temperature of the laser.
Also provided according to the invention are methods for calibrating and operating a laser wavelength monitoring apparatus and laser apparatus incorporating such laser wavelength monitoring apparatus. The methods can be practiced in accordance with the disclosure herein.
The foregoing and other objects, advantages and features of the invention will be apparent from the following description and the accompanying drawings. The drawings illustrate principles of the invention, though not drawn to scale.